Controls on platform-scale patterns of surface sediments, shallow Holocene platforms, Bahamas

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Abstract

Isolated carbonate platforms occur throughout geological history, and commonly exhibit considerable spatial variability. To evaluate the controls on the nature of sediment accumulation across the expansive, shallow platform tops, this study systematically compares and contrasts patterns in surface sediments from several shallow (<10 m) Holocene Bahamian examples. Remote-sensing data, field observations, petrographic characterization and quantitative grain-size analyses reveal the spatial patterns of sediment accumulation on Crooked–Acklins Platform and the Berry Islands Bank. Integration of these data with synoptic observations of waves, tides and currents, along with regional geochemical data, provides a means to explore the factors that influence platform-scale sedimentary patterns. These data illustrate that the platform interiors of both Crooked–Acklins Platform and Berry Islands Bank are blanketed with medium to coarse sand size sediment. Peloids are most common in the interior of Crooked–Acklins Platform, whereas the Berry Islands Bank includes more abundant composite grains. In both areas, very little mud is present, with surface sediments averaging <2% mud. Comparison of these results with published data from Little Bahama Bank, Great Bahama Bank and Caicos Platform suggest that, contrary to previous interpretations, the presence of open margins and/or brisk winds are not necessary for the occurrence of a platform top with little mud. Although the muddy sediment fraction of the interior can be suspended by elevated wave energy, wind-generated current speeds in protected platform interiors are relatively low. Instead, in parts of the platform interiors, transport and winnowing of fines is enhanced greatly by tidal currents, which carry suspended sediments off the shallow platforms, even if shielded by islands. Beyond physical influences, however, regional geochemical compilations suggest that the Bahamian tides supply highly supersaturated waters rich in dissolved oxygen to these platform interiors. This exchange is interpreted to facilitate favourable conditions for calcium carbonate precipitation in the form of ooids, marine cements and hardened peloids across vast expanses of the platform interiors. Such fundamental controls on Holocene platform-scale sediment dynamics are likely to have influenced carbonate systems through the geological record.

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